Add PBR support

[VinnyVicious]

OpenJK has recently added PBR support to their renderer: https://jkhub.org/forums/topic/7468-rend2-a-modern-renderer/

PBR models light interactions more accurately by simulating how light behaves in the real world, which allows for more detailed and lifelike materials. This would enable our engine to produce surfaces that reflect, refract, and scatter light in a way that is consistent with real-world physics, improving the overall visual quality, especially in environments with complex lighting scenarios.

It would also provide greater consistency across different lighting conditions, as materials would behave predictably regardless of the scene's lighting setup. This would reduce the time spent on tweaking material properties for different environments, streamline our asset creation pipeline, and ensure that our engine remains competitive with modern rendering standards.